1. Field of the Invention
The invention relates generally to shear wave velocity logging apparatus and, more particularly, but not by way of limitation, it relates to an improved structure for generation of torsional shear waves within a borehole for subsequent detection and processing to determine travel times, phase differences or the like of received signals thereby to derive indication of shear wave velocity in the adjacent formation.
2. Description of the Prior Art
The prior art includes various types of shear wave and compressional wave logging sonde which are operational downhole, either by transmitting energy through the borehole fluid or directly into the earth formation, to generate wave energy for subsequent detection. Designs have been various to combat problems encountered with systems utilizing the diverse forms of wave generation and encountering the differing propagation paths. That is, initial generation of P waves (compressional) into borehole fluid and the borehole wall interface result in a transformed shear wave transmitted along the borehole proximate formation which, in turn, is transformed back to compressional wave in the fluid for detection by the sonde instruments, and generation of P waves or shear waves directly into the borehole wall results in yet a different mode of energy transfer along the formation. Borehole measurements of velocity (V.sub.sh) by shear wave converted from a P wave source have proven quite difficult to interpret in any consistent manner. A detected wave train includes each of compressional, shear and Rayleigh waves in overlapping presence such that detection and definition is difficult at best.
The closest prior art known to Applicant is U.S. Pat. No. 3,354,983 in the name of Erickson et al, as assigned to the common assignee, and this patent discloses the use of shear waves as directly induced into the borehole wall for subsequent detection at spaced intervals and velocity signal processing. This device utilizes pressure feet for engaging extensible transducers into the borehole wall, but it may be readily noted that this device is not suitable for continuous logging and must be actuated and de-actuated at each positioning of the device to enable an energy propagation and detection sequence.